Biological thought is one of the best instances of the 'concepts in theories' theme that has recently become so central to the study of concept structure. But the new consensus on the embedding of concept structure in larger systems of causal-explanatory beliefs has not led to a comparable consensus on how that embedding comes about. A series of studies will explore the emergence of biological concepts in childhood with the general goal of better understanding how concepts and larger systems of explanation become intertwined. This goal leads to the following questions: do biological concepts initially emerge out of other conceptual domains such as a naive psychology or mechanics, do they emerge out of general associative processes with no initial influences of other theoretical biases, or do they appear along yet another path? Are there early influences on biological thought giving it a unique character that perseveres from preschool years on through childhood and into adulthood? How should developmental changes in biological concepts be described when invariant principles might also be present? These issues will be examined through a series of studies on a wide range of biological phenomena including: disease, inheritance, physiology and kindhood. Contrasts are drawn to superficially similar phenomena from the domains of social regularities and physical mechanical laws. It is expected that: 1. young children will see common properties and mechanisms among both animals and plants, suggesting an awareness of living things based neither on absorption into other domains, nor on pre- theoretical similarity relations, nor on animacy; 2. a small set of common principles will reappear in domain after domain, suggesting a unified conceptual approach to living things; 3. these distinctions serve as principles that give unity and coherence to conceptual change in biology; and 4. more general principles will be found concerning how concepts emerge within larger sets of causal/explanatory beliefs. Research of this sort relates to health in two ways. First, by providing insight into basic models of cognitive development, it helps us better understand boundary conditions on the normal course of mental development. Second, since biological thought is the object of study, insights gained will be relevant to issues concerning how both experts and the general public understand bio-medical phenomena.